The invention relates to a method for recording ultrasound images of moving objects, more particularly of blood vessels or organs of living things or, respectively, organisms according to the preamble of claim 1.
In order to record objects by means of ultrasound devices, an ultrasound transmitter is directed towards the object for transmission of ultrasound waves, while an ultrasound receiver receives the ultrasound waves reflected by the object. For the purpose, ultrasound heads are commonly used, which include both the ultrasound transmitter and the ultrasound receiver. In order to record a plurality of two-dimensional images, these ultrasound heads are moved along the object to be examined, while a plurality of individual images of the object are taken. The ultrasound head may be moved linearly, circularly, in an arcshape or in any other direction, for instance in a freehand manner. These images represent individual partial image areas of the object and are put together in a data processing system so that there results a three-dimensional volume image. The three-dimensional volume image consists of individual partial image areas which are xe2x80x9cpiled-upxe2x80x9d and respectively represent individual xe2x80x9clayersxe2x80x9d of the object to be examined. Due to the movement of the ultrasound head along the object to be examined, the latter is scanned in layers, wherein the number of layers per length unit determines the resolution of the resulting ultrasound image in a linear scanning process. For circular or other scanning movements, the ultrasound images are put together according to the incremental image of individual partial image areas and represented.
The movement of the sound head may be realised by a mechanism, a sound diverting means integrated in the sound head or by free hand images with magnetic or optical sensor systems for the three-dimensional assignment of the ultrasound images. Assignment of the individual xe2x80x9clayersxe2x80x9d of the object may for instance also be implemented via the method of the parallel patent application filed by applicant. The ultrasound images are put together to sense the third dimension, commonly in a data processing system which senses the video output signal or a digital output signal of the ultrasound system and evaluates it correspondingly. The ultrasound head or, respectively, the ultrasound receiver provides the corresponding signals.
Images of objects, which generate a blurred movement due to their proper movement or the movement of adjacent objects, are usually taken in synchronization with this movement. Without the synchronization with the corresponding movement of the object (stroboscopic image), there results an unsharp image or, respectively, representation of the object in each of its movement states. In an image synchronised onto the movement of the object, one volume image is generated per movement state of the respective object. One example thereof is the three-dimensional ultrasound image of the heart. The corresponding volume images of the heart successively show all stages (phases) between the contraction (systole) and the slackening of the heart (diastole). The successive representation of individual volume images of the heart corresponds to a four-dimensional representation of the heart, the fourth dimension representing the corresponding heart movement.
For the diagnosis of the individual states of the object to be examined at specific moments of the corresponding object movement, it is just one state of the object which is important. For instance, the respective organ state is important for a purposeful diagnosis at specific moments of the electrocardiogram or the respiration, the movement of the stomach or the peristalsis of the oesophagus of a patient. For instance, the stenosis (stricture) of a blood vessel is observed at the moment of diastole.
If blood vessels or organs are recorded, a single layer is recorded per movement cycle, e.g. the systole-diastole cycle (RR interval) (see FIG. 3). The image recording velocity to be achieved is very low. For images following the heart movement, i.e. the pulse frequency of the patient, about one layer is recorded per second if the pulse frequency is about 60 heart beats per minute. The image quality to be achieved is good, but the long recording times are not acceptable in most routine cases.
During the image recording of coronary vessels by means of a catheter, the time for the examination must be kept very short since the catheter closes the vessel completely or partially, which constitutes a risk factor for the patient. Here, the long recording times mentioned above are equivalent to high stress for the patient.
Movements of the patient, which lead to movement artifacts in the three-dimensional or four-dimensional image, cannot be excluded over the long time taken for the image recording, for instance for the carotid artery (swallowing, sneezing, coughing, head or throat movement etc . . . ). Then there result artifacts like blurred portions in the ultrasound images (the spatially incorrect assignment of individual ultrasound images corresponding to the partial image areas due to the organ or part of the organ being shifted or due to the assignment between organ/ultrasound head). As one solution, conventional methods use very large strokes per layer or operate with a very quick sound head movement along the object to be examined. This leads to an inhomogeneous resolution in the three-dimensional volume image in the direction of sound head movement.
Approximately 25 layers per second are recorded in a continuous image recording and with a coupling of the sound head with the video standard, e.g. PAL. However, the three-dimensional image is strongly impaired by the movement of the organ or the blood vessel during systole and during diastole. It is only in a restricted manner that the images may be used for subsequent evaluation and the doctor may possibly misinterpret the image contents (see FIG. 2).
It is the object underlying the invention to minimize the movement blur during the recording of ultrasound images of a moving object and simultaneously to shorten the image recording time. The method is to be easy to handle and shall be realised with conventional ultrasound devices and data processing systems.
The object of the invention is met by the characterising features of claim 1. Special embodiments of the invention are characterised in the subclaims.
While the ultrasound head is moved along the object for recording individual partial image areas of the moving object and produces images of the object at certain moments, which represent the individual xe2x80x9clayersxe2x80x9d of the object and are later put together to form a three-dimensional volume image in the data processing system, the movement states of the object are sensed in addition to the individual partial image areas. The images of the individual partial image areas are then assigned to the movement states of the object and put together and represented in correspondence with the movement states of the object.
More particularly, the moments for recording individual partial image areas of the object are controlled by the movement sequences of the object. For instance, the moments for recording the individual partial image areas of the moving object are fixed or controlled such that no or only few images are taken during the greatest movement of the moving object and several images are taken at moments when the object moves only little.
In order to record blood vessels or organs of an organism, no or only few images are taken during the systolic cycle and several images of individual partial image areas of the blood vessel or the organ are taken during the diastolic cycle. If the movement of the object to be examined is known, the individual partial image areas may also be recorded at discrete moments, i.e. ones which are fixedly defined. In order to record objects within an organism, which are moved by the heart beat in particular, it is suitable to control the recording moments for individual partial image areas via the signals of the electrocardiogram of the organism.
While the ultrasound head is moved along the organ of the organism, individual images of different partial image areas of the organ are taken continuously. By coupling the recording times with the ECG of the organism, the systole is left out, i.e. no images are taken over a period of about 200 ms (at a pulse frequency of 60), in order to subsequently take several images about every 120 to 200 ms in correspondence with the electrocardiogram during the diastole. This is also possible via continuous recording of images of the organ, the data processing selecting the corresponding images and processing the desired images. Thus, the corresponding images are left out during the time of the greatest movement artifacts due to pulsation whereas every fifth image is taken, for instance, at a recording velocity of 25 layers per second, for instance. Therein, the scanning movement of the ultrasound head is a continuous one. One thereby achieves an approximately homogeneous local resolution for the scan of about four layers per second.
The method may employed in the IVUS region, in particular. Here, long recording times are inacceptable in the patient""s interest (e.g. 30 mm with 0.1 mm increment at a pulse of 60 beats per minute: this corresponds to about 300 layers and results in an overall recording time of about 5 minutes).
With the method according to the invention, the recording time may be reduced to about 25% of conventional recording times (with ECG triggering). Advantageously, no images are analyzed during the systolic cycle and about 2 to 20, more particularly 2 to 10 images of individual partial image areas of the moving organ are evaluated. More particularly, the method is suited for flow representations, i.e. representations of the moving object or, respectively, individual regions of the object, which are marked with colours.
The method may also be realized in that the ultrasound head continuously records individual partial image areas of the object during the movement along the object to be examined and that, subsequently, a data processing system filters out the images of moments of greatest movement of the object. This filtering-out is also expedient with a recording method wherein the moments for recording individual partial image areas are controlled by the electrocardiogram of the organism, namely when extrasystoles, i.e. extraordinary movements of the organ or, respectively, the object to be examined occur during diastole, and these images may then be filtered out. By purposeful filtering, usable images of patients suffering from arrhythmia may also be taken, while it must be made sure that a sufficient number of images may be taken during the slackened heart phases.
If the partial image areas of an object are recorded continuously, it is possible to combine the ultrasound images respectively associated with the individual movement states of the object in a state image of the object. One respective phase of the object results in a corresponding volume image via the combination of a plurality of ultrasound images which had been taken at a specific movement state. The successive stringing-together of these volume images results in a representation of the movement sequence of the object in real time, comparable to a xe2x80x9cfilmxe2x80x9d. A so-called four-dimensional representation of the moving object is possible despite short recording times.